B. IEEE 802.11ah
However, the original WiFi standards are not suitable for IoT applications due to their frame overhead and high power consumption. Hence, IEEE 802.11 working group initiated 802.11ah task group to develop a standard that supports low overhead, power friendly communication suitable for sensors and motes . IEEE 802.11ah MAC layer features include:
Synchronization Frame: Only valid stations with valid channel information can transmit by reserving the channel medium. A station knows that it can transmit if it receives the duration field packet correctly. If it does not receive the frame correctly, then it should wait for a duration called Probe Delay. Probe Delay can be configured by the access points in 802.11ah and announced by transmitting a synchronization frame at the beginning of the transmission cycle.
Efficient Bidirectional Packet Exchange: Allowing both uplink and downlink communication between access points and the sensors is a feature in IEEE 802.11ah. This feature reduces power consumption as the sensors will go to sleep as soon as they finish their communication.
Short MAC Frame: IEEE 802.11ah reduces frame size from 30 bytes in traditional IEEE 802.11 to 12 bytes. Hence 802.11ah frame is much less overhead frame and more suitable for IoT application.
Null Data Packet: Traditional 802.11 standards had acknowledgment (ACK) frames of 14 bytes with no data. Such feature would add lots of overhead, especially for IoT. 802.11ah solves this problem by introducing a tiny signal, called preamble, which is used in place of ACKs and is much less in size.
Increased Sleep Time: As this standard is designed for power constrained devices, it allows a long sleep period and waking up infrequently to exchange data only.
Its architecture, as shown in Fig. 3, consists of a network manager, a security manager, a gateway to connect the wireless network to the wired networks, wireless devices as field devices, access points, routers and adapters. The standard offers end-to-end, per-hop or peer-to-peer security mechanisms. End to end security mechanisms enforce security from sources to destinations while per-hop mechanisms secure it to next hop only , .
Fig. 3: WirelessHART Architecture
It covers up to 30-meter distance, point-to-point communication and is suitable for small messages. It uses CSMA/CA for media access in addition to small ACK messages for reliable transmission. It follows a master/slave architecture in which the master controls the slaves, sends them commands, and handles scheduling of the whole network .
E. Bluetooth Low Energy
Its low energy can reach ten times less than the classic Bluetooth. Its access control uses a contention-less MAC with low latency and fast transmission. It adopts a master/slave architecture and offers two types of frames: adverting and data frames. The advertising frame is used for discovery and is sent by slaves on one or more of dedicated advertisement channels. Master nodes sense advertisement channels to find slaves and connect them. After connection, the master tells the slave it’s waking cycle and scheduling sequence. Nodes are awake usually only when they are communicating and they go to sleep otherwise to save their power , .
F. ZigBee Smart Energy
A coordinator controls the network and is located at the center in a star topology, the root of a tree or cluster topology and anywhere in the peer-to-peer topology. The ZigBee standard defines two stack profiles: ZigBee and ZigBee Pro. These stack profiles support full mesh networking and work with different applications allowing implementations with low memory and processing power. ZigBee Pro offers more features including security using symmetric-key exchange, scalability using stochastic address assignment, and better performance using efficient many-to-one routing mechanisms .
It is a low-cost solution that supports encryption and IPv6 addressing. It supports a master/slave architecture and is designed for burst, lightweight, asynchronous and transitive traffic and, thus, suitable for IoT. Its MAC layer features can be summarized as follows :
Filtering: An incoming frame is filtered by three processes: cyclic redundancy check (CRC) validation, a 4-bit subnet mask, and a link quality assessment. If the frame passes those checks, it can be processed, but otherwise, it will not.
Addressing: Two types of addresses are used; the unique identifier which is the EUI-64 ID and dynamic network identifier which is a 16-bit address specified by the network administrator.
Frame format: A variable length MAC frame that can be 255 bytes at maximum including addressing, subnets, estimated power of the transmission and some other optional fields.
HomePlug-AV is the basic power line communication protocol, which uses TDMA and CSMA/CA as MAC layer protocols, supports adaptive bit loading which allows it to change its rate depending on the noise level and uses orthogonal frequency division multiplexing (OFDM) and four modulation techniques.
HomePlugGP is designed for IoT applications such as smart home and smart grid applications. It is basically designed to reduce the cost and power consumption of HomePlug-AV while keeping its interoperability, reliability and coverage. Hence, it uses OFDM, as in HomePlug, but with one modulation only. In addition, it utilizes robust OFDM coding to support low rate and high transmission reliability. HomePlug-AV uses only CSMA as a MAC layer technique while HomePlugGP uses both CSMA and TDMA. Moreover, HomePlugGP has a power-save mode that allows nodes to sleep by synchronizing their sleep time and waking up when necessary .